1. Field of the Invention
This invention relates to a process for the preparation of a blocked mercaptosilane, from a metal salt of a polysulfane-containing organosilicon compound and an acyl halide or carbonyl dihalide, wherein the metal salt of the polysulfane-containing organosilicon compound is formed by reacting a polysulfane-containing organosilicon and an alkali metal, alkaline earth metal or a strong base derived from an alkali metal or alkaline earth metal. This invention also relates to the use of said blocked mercaptosilane as a coupling agent in rubber mixtures.
2. Description of Related Art
Sulfur-containing organosilicon compounds are useful as essential components in the production of silica-filled tires. A silica-filled tire provides for enhanced performance in automotive applications, specifically, improved abrasion resistance, rolling resistance and wet-skidding properties. There are a broad range of sulfur-containing organosilicon compounds that are used as coupling agents in silica-filled tires. Mercapto-containing organosilicon compounds offer superior coupling at reduced loading, however, their chemical reactivity with organic polymers results in unacceptably high viscosities during processing and premature curing. Blocked mercaptosilanes have been shown to maintain the benefits of mercapto-containing organosilicon compounds without displaying the aforenoted problems. The blocked mercapto-containing organosilicon compounds, in particular, thiocarboxylic-containing silicon compounds, can be prepared by reacting a mercapto-containing silicon compound with an acid halide. The byproduct of this reaction, hydrogen chloride, reacts with the organosilicon compound degrading the desired product, and generating chlorosilanes. These reactions with hydrogen chloride are very fast and cannot be prevented by conventional mechanical means, i.e., temperature or pressure, due to the high solubility of the hydrogen chloride in the product. Neutralization of the above noted chlorosilanes can be done using a base such as sodium alkoxide, or propylene oxide, but degradation of the product and/or an undesirable mixture is obtained making this approach undesirable.
Another process used previously, is to neutralize the hydrogen chloride, in situ, using an acid acceptor, i.e., tertiary amine, see U.S. Provisional Patent Application No. 60/423,577 filed Nov. 4, 2002; but this requires a stoichiometric amount of the amine that reduces batch yield and affords a large amount of an undesirable salt that must subsequently be removed, see U.S. Pat. No. 6,229,036. As is already known, tertiary amine salts are difficult to remove due to their solubility, and conventional filtration methods are mechanically intensive and often lead to poor yields. Furthermore, further processing of the filter cake adds additional costs, such as the disposing of the tertiary amine and the hydrogen chloride salt in itself, which poses significant environmental issues.
As has been shown, the reaction of a metal salt of a mercapto-containing organosilicon compound and an acid halide generates the desired blocked mercaptosilane and a metal halide salt, see U.S. Pat. No. 6,414,061 the contents of which are incorporated herein by reference; but in addition to the previously mentioned difficulties, mercapto-containing organosilicon compounds are expensive making their widespread use prohibitive. Therefore, there is an interest in developing a blocked mercaptosilane using a process that is inexpensive and does not provide for the chemical and environmental concerns noted above. There are a number of known methods to cleave a sulfur-sulfur bond, i.e., the use of bases such as amines, phosphines, metal cyanides, metal hydrides and alkali metals, however, phosphines and metal hydrides are expensive and metal cyanides offer a host of safety concerns.
Polysulfane-containing silicon compounds are inexpensive and widely available, in addition to their affordability, the metal halide byproduct does not react with the product blocked mercaptosilane nor do polysulfane-containing silicon compounds have the environmental concerns of a tertiary amine halide salt. Alkali metals are both safe and inexpensive. The reaction of an alkali metal with a polysulfane-containing organosilicon compound to generate the metal salt of the polysulfane-containing organosilicon compound, affords the desired acid acceptor, in situ, that can be used to produce the desired blocked mercaptosilane compound and a metal halide salt.
Additionally, the use of an aqueous wash of the product thereby minimizes the aforementioned costs and difficulties of removing the metal salt. Removing the metal halide salt by means of either filtration or by use of a centrifuge requires intensive mechanical unit operations and capital investment, whereas an aqueous wash requires neither but results in a two-phase system where one phase contains the blocked mercaptosilane and the second phase contains an aqueous solution of the metal halide. The primary hazard with this method is the potential for hydrolysis of the blocked mercaptosilane and for organofunctional silanes. However, through the presence of metal halide the ionic character of the aqueous phase is increased and thereby minimizes any hydrolysis reaction, see U.S. Pat. No. 6,294,683.
It is an object of the invention to provide a process for preparing a blocked mercaptosilane for use as a coupling agent, which process minimizes the production of byproducts that react with the blocked mercaptosilane, does not require neutralization or filtering and is commercially affordable.
In keeping with this and other objects of the invention there is provided a process for the manufacture of a blocked mercaptosilane comprising:
(a) reacting at least one polysulfane-containing organosilicon compound of the general formula:
(R13SiG)2Sn
xe2x80x83in which each R1 is independently methoxy, ethoxy or alkyl of from 1 to about 6 carbon atoms, provided, that at least one R1 group is methoxy or ethoxy, G is an alkylene group of from 1 to about 12 carbon atoms and n is from 2 to about 8, with at least one alkali metal, alkaline earth metal or a basic derivative of an alkali metal or alkaline earth metal to provide the corresponding metal salt of the polysulfane-containing organosilicon compound and;
(b) reacting the metal salt of the polysulfane-containing organosilicon compound with an acyl halide or carbonyl dihalide to provide a blocked mercaptosilane.
In contrast to the process described in aforementioned U.S. Pat. No. 6,414,061 the process of this invention makes it possible to produce a blocked mercaptosilane from readily available polysulfane-containing organosilicon compounds. This results in a high purity blocked mercaptosilane that does not require neutralization or filtering to remove the byproduct metal halide that is formed upon reaction of the metal salt of the polysulfane-containing organosilicon compound with the acyl halide or carbonyl dihalide.
A further object of this invention is to provide a process that involves the use of an aqueous wash of the final product solution, which unlike the distillation step required in aforementioned U.S. Pat. No. 6,414,061, is a more expedient and efficient way to separate the product blocked mercaptosilane from the metal halide byproduct.
In the process of this invention, a polysulfane-containing organosilicon compound of the formula (R13SiG)2Sn in which R1, G and n have the aforestated meanings, and an alkali metal, alkaline earth metal or a strong base derived from the alkali metal or alkaline earth metal, can be considered to react to form a metal salt of the polysulfane-containing organosilicon compound in accordance with the reaction (illustrated for an alkali metal such as sodium):
Metal+(R13SiG)2Snxe2x86x92(R13SiG)2S-Metal
The metal salt of the polysulfane-containing organosilicon compound and a reactive halide such as an acyl halide or carbonyl dihalide, e.g., of the formula R2C(O)X in which R2 and X have the aforestated meanings, can then be considered to react to form the product blocked mercaptosilane and a metal halide byproduct in accordance with the reaction:
(R13SiG)2S-Metal+R2C(O)Xxe2x86x92R2C(O)SGSiR13+Metal-X
Useful polysulfane-containing organosilicon compounds include, for example, bis[(triethoxysilyl)propyl]polysulfane, bis[(methyldiethoxysilyl)propyl]polysulfane, bis[(triethoxysilyl)isobutyl]polysulfane, bis[(methyldiethoxysilyl)isobutyl]polysulfane, bis[(trimethoxysilyl)propyl]polysulfane, bis[(methyldimethoxysilyl)propyl]polysulfane, bis[(trimethoxysilyl)isobutyl]polysulfane, and bis[(methyldimethoxysilyl)isobutyl]polysulfane.
The polysulfane-containing organosilicon compound is reacted with an alkali metal, alkaline earth metal or a strong base derived from an alkali metal or alkaline earth metal. Useful alkali metals, alkaline earth metals and basic metal derivatives include, for example, lithium, sodium, potassium, magnesium, calcium, lithium hydride, sodium hydride, potassium hydride, magnesium hydride, calcium hydride, sodium methoxide, sodium ethoxide, potassium methoxide, potassium ethoxide, and mixtures thereof.
The resulting polysulfane-containing organosilicon compound metal salt is combined with an acyl halide or carbonyl dihalide, e.g., of the general formula R2C(O)X supra in which R2 is halogen or alkyl, alkenyl, aryl, alkaryl or aralkyl of up to about 18 carbon atoms and X is halogen, to produce a blocked mercaptosilane. Useful acyl halides include acetyl chloride, propanoyl chloride, butanoyl chloride, pentanoyl chloride, hexanoyl chloride, heptanoyl chloride, octanoyl chloride, 2-ethylhexanoyl chloride, lauroyl chloride, oleoyl chloride, octyl chloroformate, adipoyl chloride, phenylacetyl chloride, benzoyl chloride, terephthaloyl chloride, and phenyl chloroformate. Useful carbonyl dihalides include carbonyl dichloride (phosgene), diphosgene, triphosgene, thiophosgene, and oxalyl chloride.
The blocked mercaptosilane product obtained by the foregoing process conforms to the general formula R2C(O)SGSiR13 wherein, R1, R2, and G have the aforesaid meanings. Specific blocked mercaptosilanes include, for example, 2-triethoxysilyl-1-ethyl thioacetate; 2-trimethoxysilyl-1-ethyl thioacetate; 2-(methyldimethoxysilyl)-1-ethyl thioacetate; 3-trimethoxysilyl-1-propyl thioacetate; triethoxysilylmethyl thioacetate; trimethoxysilylmethyl thioacetate; triisopropoxysilylmethyl thioacetate; methyldiethoxysilylmethyl thioacetate; methyldimethoxysilylmethyl thioacetate; methyldiisopropoxysilylmethyl thioacetate; dimethylethoxysilylmethyl thioacetate; dimethylmethoxysilylmethyl thioacetate; dimethylisopropoxysilylmethyl thioacetate; 2-triisopropoxysilyl-1-ethyl thioacetate; 2-(methyldiethoxysilyl)-1-ethyl thioacetate; 2-(methyldiisopropoxysilyl)-1-ethyl thioacetate; 2-(dimethylethoxysilyl)-1-ethyl thioacetate; 2-(dimethylmethoxysilyl)-1-ethyl thioacetate; 2-(dimethylisopropoxysilyl)-1-ethyl thioacetate; 3-triethoxysilyl-1-propyl thioacetate; 3-triisopropoxysilyl-1-propyl thioacetate; 3-methyldiethoxysilyl-1-propyl thioacetate; 3-methyldimethoxysilyl-1-propyl thioacetate; 3-methyldiisopropoxysilyl-1-propyl thioacetate; 1-(2-triethoxysilyl-1-ethyl)-4-thioacetylcyclohexane; 1-(2-triethoxysilyl-1-ethyl)-3-thioacetylcyclohexane; 2-triethoxysilyl-5-thioacetylnorbornene; 2-triethoxysilyl-4-thioacetylnorbornene; 2-(2-triethoxysilyl-1-ethyl)-5-thioacetylnorbornene; 2-(2-triethoxysilyl-1-ethyl)-4-thioacetylnorbornene; 6-triethoxysilyl-1-hexyl thioacetate; 1-triethoxysilyl-5-hexyl thioacetate; 8-triethoxysilyl-1-octyl thioacetate; 1-triethoxysilyl-7-octyl thioacetate; 6-triethoxysilyl-1-hexyl thioacetate; 1-triethoxysilyl-5-octyl thioacetate; 8-trimethoxysilyl-1-octyl thioacetate; 1-trimethoxysilyl-7-octyl thioacetate; 10-triethoxysilyl-1-decyl thioacetate; 1-triethoxysilyl-9-decyl thioacetate; 1-triethoxysilyl-2-butyl thioacetate; 1-triethoxysilyl-3-butyl thioacetate; 1-triethoxysilyl-3-methyl-2-butyl thioacetate; 1-triethoxysilyl-3-methyl-3-butyl thioacetate; 3-trimethoxysilyl-1-propyl thiooctanoate; 3-triethoxysilyl-1-propyl thiopalmitate; 3-triethoxysilyl-1-propyl thiooctanoate; 3-triethoxysilyl-1-propyl thiobenzoate; 3-triethoxysilyl-1-propyl thio-2-ethylhexanoate; 3-methyldiacetoxysilyl-1-propyl thioacetate; 3-triacetoxysilyl-1-propyl thioacetate; and 2-methyldiacetoxysilyl-1-ethyl thioacetate.
The reaction of the polysulfane-containing organosilicon compound with alkali metal, alkaline earth metal, or basic derivatives of alkali metal or alkaline earth metal is carried out in mole equivalents of from about 1:1 to about 1:10, and preferably from about 1:2.0 to about 1:2.5. The reaction of the polysulfane-containing organosilicon compound metal salt with acyl halide can be carried out in the range of 1.25:1 to about 1:1 or with a carbonyl dihalide in the range of from about 2.25:1 to about 2:1 mole equivalents.
The reaction between the polysulfane-containing organosilicon compound and the alkali metal, alkaline earth metal or basic metal derivative is conducted at a range of from about ambient temperature to about the melting temperature of the metal or metal derivative used. It is preferably conducted at a temperature wherein the metal used is in a liquid state to increase its surface area such as, for example, from about 25xc2x0 to about 150xc2x0 C. and preferably in the range of from about 80xc2x0 to about 120xc2x0 C. The subsequent reaction of the metal salt of the polysulfane-containing organosilicon compound and acyl halide or carbonyl dihalide can be carried out at a temperature of from about ambient temperature to about the boiling point of the solvent used; and preferably the temperature is from about 10xc2x0 to about 50xc2x0 C. The aqueous wash of the product blocked mercaptosilane and metal halide is conducted in a range of from about 4xc2x0 to about 100xc2x0 C. and preferably from about 10xc2x0 to about 50xc2x0 C. In addition, the entire process or any step therein, may be conducted at ambient, elevated or reduced pressure.
The entire process of this invention or any step therein, can be conducted in a solvent. Useful solvents can be, for example, any aromatic compound, such as, toluene, benzene, xylene, and any hydrocarbon solvent, such as, hexane, heptane, isooctane and octane.
The following examples are illustrative of the process of this invention.
All operations were performed under a nitrogen atmosphere. Silquest(copyright) A-1589 (bis(triethoxysilylpropyl)disulfane), Silquest(copyright) A-15304 xe2x80x9cmore purified disulfide then Silquest(copyright) A-1589xe2x80x9d (bis(triethoxysilylpropyl) disulfane), Silquest(copyright) A-1289 (bis(triethoxysilylpropyl)tetrasufsulfane), toluene, and sodium were used as received without further purification. Deionized water was used as obtained. All GC data is expressed in weight mass % (wt/wt) and obtained from the GC Lab using a Hewlett-Packard 5890 Series II gas chromatograph. The following abbreviations and tradenames (with their descriptions) appear in the examples: